1. Field of the Invention
The present invention relates to a high-strength stabilizer steel for vehicles which is mainly used for vehicles and a method of producing the same, particularly to a stabilizer having a high tensile strength of 1100 MPa or more and excellent corrosion resistance and low-temperature toughness.
2. Description of the Related Art
A stabilizer bar (hereinafter simply referred to as a stabilizer) is a spring component having a function in which, for running stability of vehicles, particularly during rotating, the stabilizer is twisted and displaced in an opposite direction to horizontal shaking loads transmitted from wheels via a suspension mechanism and an excessive tilt of the vehicle body is prevented by reducing the horizontal shaking loads working on the suspension mechanisms on either side. Since it is exposed to such a usage environment, the frequency of repeated load which is applied to the stabilizer is generally lower than that of a coil spring. On the other hand, the load applied to the stabilizer is accompanied by acceleration and thus sufficient strength, durability, and toughness higher than that of the coil spring are required for a steel which is used as a material. A carbon steel such as S48C and a spring steel such as JIS SUP9 have been used for conventional stabilizers. For example, a process of manufacturing these steels includes cutting a hot-rolling steel material into a predetermined size, performing a hot bending/molding process, adjusting to a predetermined strength by a heat-treating process for hardening and tempering, and then, subjecting the surface to shot peening, and finally coating for corrosion protection.
Recently, a demand of reduction in weight based on an increase in strength of underbody components aiming at the fuel consumption improvement of vehicles has been further increased. On the other hand, in the stabilizer field, a high-strength stabilizer of 1000 MPa or more has been developed. For example, a non-heat treated steel for spring having a strength of 120 to 150 kgf/mm2 obtained by adding a proper amount of Ti, Nb, and B, particularly an amount of 0.08% or more of Ti and Nb is proposed in Patent document 1.
Patent document 1: Jpn. Pat. Appln. KOKAI Publication No, 3409277